During the drilling of a sub-surface well, it is typically desirable that the pressure conditions in the borehole are controlled. This may be to reduce the risk of blow-outs or well kicks where a sudden build-up and release of pressure may occur deep in the borehole and may be communicated back to a drilling rig at the surface.
Such a well is typically drilled using drilling apparatus comprising drill pipe fitted with a drill bit for penetrating into a subsurface, for example by rotation of the drill pipe from a surface platform. A drilling fluid is conveyed through the inside of the drill pipe and delivered into the borehole as drilling progresses. Drilling fluid is returned back up toward the surface through an annular space outside of the drill pipe, between the drill pipe and the wall of the borehole. The drilling fluid may help to lubricate and cool the drill bit and may help carry drill cuttings and debris out of the well. The drilling fluid also plays an important role in controlling the fluid pressure in the borehole, and is often selected to have a density with the aim of providing a particular pressure in the borehole.
Typically, it is desired that the pressure in the borehole be controlled to be higher than the pressure of the formation (overbalanced drilling). This helps to prevent influx of fluids from the formation and collapse of the formation into the borehole during drilling. More specifically, the pressure in the borehole may be sought to be higher than the pore fluid pressure but less than the fracture pressure of the formation. In some situations, depending on lithology and burial conditions of a formation, the fracture pressure may not be much higher than the pore pressure, resulting in a narrow pressure margin within which to maintain borehole pressure in order to drill the well in overbalanced conditions.
In such situations, accurate control of the pressure conditions in the borehole is required. The drilling fluid may be selected such that a desired pressure in the borehole can be achieved. A difficulty is that the drilling fluid in the borehole picks up cuttings or debris from the borehole, such that the density of the drilling fluid in the borehole may differ from that delivered through the drill pipe.
In typical offshore drilling, the drilling fluid is passed through a drill pipe from a floating drilling vessel to the bottom of the well, and the drilling fluid is returned from the borehole through a passage between the drill pipe and a drilling riser. The pressure in the borehole at the penetration depth of the formation includes the hydrostatic pressure imparted by the drilling fluid extending from the bottom of the borehole all the way to the surface (top of the drilling riser) plus the equivalent circulating density (ECD) of the drilling fluid when it is circulating.
In deep water drilling operations, this may pose difficulties because the drilling fluid extends within the drilling riser a significant distance through the water column. In particular, it can mean highly constrained pressure margins between pore and fracture pressures of the formation, and it can be problematic to control the pressure in the borehole accordingly.
For deep water drilling, it has been proposed therefore to use dual gradient drilling methods, where the riser has a lower density fluid above a certain depth. At that depth, a seal is formed around the drill pipe, separating the lower density fluid above from the drilling fluid below. Such seals are typically called rotating control devices (RCDs) although such seals are not always configured to rotate. Such an arrangement results in a first pressure gradient with depth in the riser annulus for the interval spanning the lower density fluid, and a greater pressure gradient with depth in the annulus below that level. It is documented that such an approach can help to expand the drilling length within given pressure margins.
There have also been proposed methods where an RCD seal is provided in a similar way, but there is no riser fitted above the seal (for example, as in deep-water riserless drilling). A dual gradient effect is achieved, but the hydrostatic pressure above the seal is given by that provided by the seawater above the seal.
In dual gradient and/or riserless drilling configurations such as these, a subsea pump is required to lift the return flow of drilling fluid back to the surface through a separate mud return line. The subsea pump is typically placed at or near the same depth as that of the seal.
For this purpose, it has been proposed to use subsea pump in the form of a positive displacement pump that is driven by seawater, using the hydrostatic pressure of seawater plus pump pressure from rig-based pumps. The minimum drive pressure may be the hydrostatic pressure of seawater at the pump or above the seal.
An example subsea positive displacement pump is described in the patent publication U.S. Pat. No. 6,904,982.